1. Field of the Invention
The present invention relates to a driving apparatus, a power output apparatus, and to a control method applicable to these apparatuses.
2. Description of the Related Art
A power output apparatus provided with a capacitor connected between a positive pole bus and a negative pole bus of an inverter circuit for applying a three-phase alternating current to an electric motor and a direct-current power supply connected between the positive pole bus or the negative pole bus of the inverter circuit and a neutral point of an electric motor has been proposed in, for example, Japanese Laid-Open Patent Publications No. Hei 10-337047 and Hei 11-178114. All these publications require that the apparatus have the functions of charging the capacitor and driving the electric motor by realizing, by time shearing, the operation of making a circuit composed of coils in each phase of the electric motor and switching devices of the inverter circuit function as a booster chopper circuit for boosting the voltage of the direct-current power supply to charge the capacitor and the operation of making the inverter circuit function as an original circuit for driving the electric motor by the use of the capacitor voltage.
However, in such a power outputting apparatuses, it is necessary to limit the voltage input to the inverter circuit, i.e. the voltage between the terminals of the capacitor, to a voltage within a range from the voltage of the direct-current power supply to about three times the voltage of the power supply when the difference between the voltage of the positive and the negative buses viewed from the neutral point of the electric motor is considered. The electric motor can effectively be driven if the input voltage to the inverter circuit can be controlled according to the state of the electric motor. However, driving efficiency of the electric motor is limited when the range of the input voltage is limited.
A driving apparatus, a power output apparatus, and their control method all have an object of controlling an input voltage to their inverter circuits over a wide range. Moreover, the driving apparatus, the power output apparatus, and the control method also aim to more efficiently drive the electric motor.
In the driving apparatus of the present invention, an input voltage of its inverter circuit is determined by the neutral point electric potential of each winding group to which a secondary power supply of the driving apparatus is connected, while the neutral point electric potential of a winding group which is a load of a polyphase alternating current and to which the inverter circuit is connected can be varied by the inverter circuit. The neutral point electric potential of a winding group which is a load of a polyphase alternating current and to which the inverter circuit is not connected can also be varied by neutral point electric potential control means. The input voltage of the inverter circuit can thereby be freely set regardless of the voltage of the secondary power supply.
In such a driving apparatus of the present invention, the neutral point electric potential control means may be means including an inverter circuit connected with a load of a polyphase alternating current according to the control.
Moreover, in a driving apparatus of the present invention, the load of the plural loads of the polyphase alternating current may be loads equipped by a single electrical apparatus or loads equipped by a plurality of electrical apparatuses.
Moreover, in a power output apparatus of the present invention, by control of two inverter circuits using common positive and negative pole buses, electric power is transferred between a first power supply connected between the positive pole bus and the negative pole bus and a second power supply connected between the neutral points of two star connection coils of an electric motor and polyphase alternating current electric power is supplied to the two star connection coils. Consequently, the voltage between the positive pole bus and the negative pole bus can be controlled over a wide range, and the current to be supplied to the two star connection coils can be controlled. Consequently, the difference in electric potential between the positive pole bus and the negative pole bus, i.e. an input voltage into the two inverter circuits, can be controlled, and the electric motor can therefore be driven more efficiently.
In a first power output apparatus of the present invention, accumulating means capable of being charged and discharged may be used as the first power supply. Accumulating means with a small capacity may be used because the voltage between the terminals of the accumulating means can be controlled.
Moreover, it is preferable to control the voltage of the first power supply by separately controlling percentage modulations, which are ratios of on-periods of the upper side switching devices and the lower side switching devices of the two inverter circuits. In such a case, when the percentage modulation in one of the two inverters is designated d1, the percentage modulation in the other of the two inverters is designated d2, an output voltage of the second power supply is designated Vb, and an output voltage of the first power supply is designated Vc, the percentage modulations in the two inverters can severally be controlled using the formula,
Vc=Vb/(d1xe2x88x92d2).
As a result, the voltage values of the first power supply can easily be controlled.
Moreover, when there is dead time when all of the switching devices in a plurality of series connections of the upper side switching devices and the lower side switching devices are turned off, it is preferable that the formula be corrected in consideration of the dead time.
Moreover, it is also preferable to provide the two star connection coils correspondingly to one rotor to constitute an electric motor. In such a case, in order to output a desired torque from the electric motor and so as to maintain target voltages of the first power supply, it is preferable to control the switching of the plural switching devices in the two inverter circuits by making the phase difference between each polyphase alternating current electric power to be supplied to the two star connection coils of the electric motor equal to the phase differences between the two star connection coils.
Moreover, it is also suitable to provide the two star connection coils correspondingly to severally separated rotors and to form two separated motors. In this case, it is preferable to control the switching of the respective plural switching devices in the first inverter circuit and the second inverter circuit so as to output desired torque from the first electric motor and to output desired torque from the second electric motor and so as to hold the voltage of the first power supply to be an object voltage.
Moreover, it is preferable to decrease the maximum value of a current amplitude to be supplied to one of the star connection coils and to add a current corresponding to the decrease amount to a current to be supplied to the other of the star connection coils. Thereby, the maximum value of the current amplitude can be decreased, and the rated voltage and the like of the inverter can be decreased.
Moreover, it is preferable to determine the decreased amount of one of star connection coil and the added amount of the other star connection coil under the condition such that the output torque of the electric motor does not vary. Thereby, influences to the output torque can be eliminated.
Moreover, it is preferable to determine the decreased amount of one of star connection coil and the added amount of the other star connection coil under the condition such that no influences to the current flowing between the neutral points of the two star connection coils occur. Thereby, undesirable effects on the voltage control of the first power supply can be prevented.
It should be understood that, in the power output apparatus, the xe2x80x9celectric motorxe2x80x9d includes dynamotor capable of generating electrical energy.